The instant invention relates to an optical processor or furnace for delivering optical energy of certain predetermined characteristics to a process-wafer or semiconductor wafer. Specifically, the optical furnace uses a bank of lights for annealing the process wafer as used in semiconductor device fabrication. In the past, the distribution of light in a conventional processor displays non-uniformities reminiscent of the spatial distribution of the lamps. The instant invention uses a diffuser in combination with a quartz box to direct the light to process the wafer. The invention also uses a sensor or detector for measuring the light to monitor optical energy delivered and transmitted into the process wafer, thus the optical energy delivered in watts/cm.sup.2 is used to control the process. The detected optical energy is also used to determine the steady-state temperature of the wafer during processing.
The improved optical processor is used during the fabrication of semiconductor devices, and as such is important for the microelectronic and photovoltaic industries. In the semiconductor industry electronic components and circuitry are formed in a thin wafer of semi-conductive material. During processing, the wafer at various stages is subjected to heat treatment or annealing.
Electric ovens or furnaces have been used for heat treatment. However, such ovens are expensive and slow to use and typically result in distortion of the wafer due to impurity redistribution. Such distortion occurs through impurity diffusion into the wafer since impurities permeate through the hot walls of the furnace or oven.
Conventional rapid thermal annealers are now used extensively in the industry. Such annealers use an optical source powerful enough to quickly raise the temperature of the semiconductor device. However, such annealers, by raising the speed of heating, also increase non-uniform heating of the process wafer. Also, the lamp separation as well as the output distribution from the lamp contributes to such non-uniform heating of the process wafer.
Such prior art processes for fabricating semiconductors have introduced several problems that up to now have been lacking in solutions. Conventional rapid thermal annealers typically introduce non-uniform light which creates undesirable light patterns in the processwafer, and causes breakage of and/or fissures in, any glass used in the furnace. If the process-wafer is not heated uniformly, such temperature variations have a residual, uncontrollable effect on the resulting electrical characteristics of the processed wafer. These undesirable electrical characteristics are the result of non-uniform electrical activation, and/or the result of defects and distortions in the wafer caused by the stress associated with the uneven light processing.
Also included in prior art processes is a feedback system for measuring the temperature in the particular furnace used. Such a feedback system typically uses the measured temperature to adjust the amount of power delivered by the electrical heat sources. Generally such heat measurements are taken off the wafer itself by a thermocouple which presents time delays in correcting the electrical sources. Other means of temperature measurement, such as pyrometers, have also been used.
In the past semiconductor wafers have been heat processed in a quartz sleeve or envelope or under a quartz cover. Such a feature is shown in U.S. Pat. No. 4,540,876 to McGinty, U.S. Pat. No. 4,794,217 to Quan et al, and U.S. Pat. No. 5,179,677 to Anderson et al. The use of a silica box is also shown in U.S. Pat. No. 4,550,245 to Arai et al. Such boxes or envelopes were used to even the light dispersement, and to allow the introduction of cooling gases. They also shield the process-wafer from may cooling atmosphere. It was thought that the use of quartz or silica allowed the excess optical energy to pass therethrough. Such boxes or envelopes are formed of high temperature melting material as shown in the references.
Several problems not recognized or addressed by the references are the subject of the instant application, however. Notably the references do not discuss the possibility of excessive absorption of light by the quartz box or envelope. This absorption of such high intensity light over a period of time results in cracking and ultimately breakage of the quartz box or envelope. Replacement of such quartz boxes and envelopes is expensive, and interrupts the manufacturing process.
The absorption also results in heating of the quartz or silica box or envelope. This heating provides impurity migration to or in the process-wafer, and such impurities are detrimental to the final product. Energy absorption by the quartz box or envelope effects the total energy absorption of the semiconductive wafer.
The use of parallel quartz plates between the light sources and a process-wafer is shown in U.S. Pat. No., 4,680,447 to Mahawili, and U.S. Pat. No. 4,101,759 to Anthony et al. In both cases the use of parallel plates was directed to cooling concerns. In the 4,680,447 patent, temperature controlling fluid is passed between the two plates to prevent excessive heating. In the 4,101,759 patent the second plate is used to minimize free convection cooling of the semiconductor wafers. Thus the references are not concerned with damage of the quartz plate due to excessive heat absorption. Divitrification and cracking as a precourse to breaking of the quartz is not discussed by either of the references.
A diffuser to disperse the light is shown in U.S. Pat. No. 4,755,654 to Crowley et al. The use of a quartz envelope or box in combination with the diffuser is not shown. The reference uses a lens as a diffuser to redistribute the light from the lamp and the reflector on a wafer. The diffuser can also act like a diffraction grating to expose the semiconductor to a particular non-uniform light pattern. Unlike the instant invention the patent discloses the use of a box with a reflective surface for wafer support. The use of a quartz box or muffle is not contemplated by the patent.
One solution for the problem of uneven heating is shown in U.S. Pat. No. 4,469,529 to Mimura. This solution requires the use of supplemental circumferential heating. This is a more expensive and complicated solution than that of the instant invention.
A second feature of the instant invention is a novel feedback light sensing system. The feedback system of U.S. Pat. No. 4,857,704 senses radiation reflected off the back of the water. A prism and optical fiber is used to direct the radiation. This required structure adds to the expense of the feedback system, and requires the additional step of receiving reflected radiation from the probe.
A temperature sensing feedback system is shown in U.S. Pat. No. 4,761,538 to Chiba et al. To detect the temperature a sensor such as a thermocouple is provided in the heater furnace. Infrared detectors are used to measure laser beam power for determining real-time wafer temperature in U.S. Pat. No. 4,956,538 to Moslehi. Again an extensive optical system is required to perform the sensing.
An arc lamp for semiconductor heating is shown in U.S. Pat. No. 4,820,906 to Stultz. The patent also specifically discloses the use of quartz to surround the lamp electrodes.